1. Field of the Invention
The present invention relates to an intermittent drive control apparatus of motor suitable for intermittent drive of a capstan motor of a video tape recorder (VTR).
2. Related Art Statement
In the conventional VTR of helical scanning scheme, there is sometimes adopted a slow motion playback system using such an intermittent slow scheme that tape stop and tape drive are alternately repeated. In this scheme, the capstan motor is rotated intermittently and tape is fed intermittently. Slow motion playback with noise occurrence suppressed is possible in this scheme.
Intermittent feed for such slow motion playback will now be described.
For intermittent feed of tape, a forward/reverse rotation command is supplied to a motor drive circuit of the capstan motor. In addition, since the motor is started until a predetermined rotational speed is attained, a current limit command is given so as to raise the rotational speed rectilinearly. Next when the motor attains a predetermined rotational speed, to maintain the speed, a speed control is performed by an error command. If a reproduced control pulse is obtained in the middle of an interval of this fixed speed, a reverse rotation command is given after a preset tracking adjustment time interval and a braking current is supplied. The braking current is also kept at a fixed value by a current limit command so as to lower the rotational speed of the motor at a fixed ratio. The rotational speed of the motor gradually lowers, and the rotation becomes zero. If rotation in the reverse rotation direction is subsequently detected, then a forward rotation command is given. As a result, continuation of reversal caused by inertia of the motor is prevented. (This is called re-acceleration.) Complete stop is thus obtained.
The intermittent drive of the capstan is implemented by the above described system. Slow motion playback is conducted so as not to generate noise. In the above described system, however, it is necessary to finish the braking and conduct re-acceleration operation at a time point when the rotation becomes zero. When an accuracy in detection of this zero point is low, tape is damaged at a time of tape travelling. Therefore, zero point detection is conducted by detecting a phase relation by means of two capstan rotational speed detection signals differing in phase as means for improving the zero point detection accuracy.
FIG. 3 shows a related technique of a VTR which makes intermittent feed possible. FIG. 3 is a block diagram showing invention disclosed in Japanese Patent Application Laid-Open 7-59389.
A capstan shaft 11 of the VTR is capable of pressing magnetic tape 13 against a pinch roller 12 and driving the magnetic tape 13 to make it travel. The capstan shaft 11 is integral with a rotary shaft of the capstan motor 15. Rotation of the capstan motor 15 is detected by bi-phase frequency detectors (FGa, FGb).
The frequency detectors FGa and FGb are coils disposed at such a distance as to generate a phase difference of 90 degrees between them. Detected outputs of the frequency detectors FGa and FGb are supplied to waveform shaping circuits 16 and 17, respectively. For a control track of tape 13, a control head 18 is disposed. Thereby, a control signal can be recorded and reproduced. A reproduced control signal is inputted to a waveform shaping circuit 19.
The detection signal FGa shaped in waveform to a rectangular wave by the waveform shaping circuit 16 is supplied to a speed detector 21, and used to detect the rotational speed of the capstan motor 15. The detection signals FGa and FGb of rectangular waves outputted from the waveform shaping circuits 16 and 17 are supplied to a rotational direction detector 22, and used to detect the rotational direction of the capstan motor 15.
A speed error signal fed from the speed detector 21 and a reverse rotation detection signal fed from the rotational direction detector 22 are supplied to a capstan controller 23. In addition, a control signal fed from the waveform shaping circuit 19 and tracking information fed from an external memory or the like are also inputted to the capstan controller 23.
By using these kinds of input information, the capstan controller 23 controls a motor current of a motor drive circuit 24 and drives the capstan motor 15 intermittently. Control information supplied to the motor drive circuit 24 includes forward/reverse rotation command information, current limit value command information, and error information.
FIGS. 4A–4G show signal waveforms of various locations at the time of intermittent drive.
It is now assumed that start of the motor is begun at time t0. At this time, a motor current FIG. 4F is supplied with a fixed value by current limit value command information FIG. 4E so as to increase the rotation at desired acceleration. A motor speed FIG. 4G increases at fixed acceleration as shown in FIG. 4G. When the motor speed FIG. 4G arrives at a desired value W0 (time t1), rotational speed control is applied in order to keep the fixed speed.
An interval between time t0 and t1 is called start interval (a start time interval is Ts). Shift to the fixed speed state is conducted on the basis of the speed information fed from the speed detector 21, and error command information FIG. 4A is supplied to the motor drive circuit 24. As a result, the motor current changes, resulting in a fixed speed state.
Maintenance of the constant speed state is achieved by conducting speed control so as to make the frequency of the rotation detection output (FGa) constant. This interval (interval between the time t1 and time t2) is called fixed speed interval (where a fixed speed time interval is TF).
When the control signal FIG. 4B is detected in the fixed speed interval (at, for example, t5), the speed W0 is kept further for a time interval (TD) on the basis of tracking information, and then shift to braking operation is conducted (time t2). The braking operation is conducted by supplying forward/reverse rotation command information FIG. 4D to the motor drive circuit 24 and switching the motor current over to an opposite direction. At this time, the motor current FIG. 4F is prescribed (to become IB in FIG. 4F) by the current limit value command information FIG. 4E so as to decrease the rotational speed of the motor at a fixed rate. If braking operation were kept, the motor would conduct reverse rotation operation after stop. The moment an opposite direction rotation detection output (reverse rotation detection signal FIG. 4C is obtained from the rotation direction detector 22 (at time t3), therefore, the forward/reverse rotation command FIG. 4D is changed so as to order a forward rotation. At the same time, a current is applied in the forward rotation direction again for a short time in order to cancel the rotational inertia of the motor. As a result, rotatory power of the forward rotation direction is generated. The reverse rotation energy is thus absorbed completely, and complete stop is obtained. This is so-called re-acceleration. A re-acceleration time interval is TR, which is an interval between t3 and t4. (As a matter of fact, the motor is in the stop state.)
For implementing such accurate intermittent drive as not to generate noise on the screen, as above-described, it is necessary to conduct detection of timing for execution of re-acceleration, i.e., detection such timing that the rotation becomes zero (zero point detection) at high accuracy. For this reason, two capstan rotation detection signals having a phase difference of 90 degrees are used in an example of the aforementioned Japanese Patent Application Laid-Open 7-59389.
In other words, in the forward rotation, FGb is detected 90 degrees behind the rotation detection signal FGa. On the other hand, in the reverse rotation direction, FGb is detected 90 degrees before the rotation detection signal FGa. By detecting s phase relation between FGa and FGb, therefore, it becomes possible to determine whether the motor is conducting the forward rotation or the reverse rotation at the present time. It thus becomes possible to detect a zero point at high accuracy.
In this case, however, two rotation detection systems, i.e., two motor speed detectors, two detection signal amplifiers and two wave form shaping circuits are needed, resulting in a remarkably increased cost.
In this way, in an intermittent drive control apparatus of motor adopted in an intermittent slow playback system, it is necessary to improve the zero point detection accuracy. Therefore, zero point detection is conducted by using two capstan rotational speed detection signals differing in phase. Accordingly, two motor speed detectors, two detection signal amplifiers and two wave form shaping circuits are needed. This results in a problem that the apparatus scale is large and the cost is increased.
Therefore, an object of the present invention is to provide an intermittent drive control apparatus of motor capable of reducing an apparatus scale and suppressing a cost increase by making possible high accuracy zero point detection using only one capstan rotation detection signal.